When comparing Ruby vs F#, the Slant community recommends Ruby for most people. In the question“What is the best programming language to learn first?”Ruby is ranked 7th while F# is ranked 28th. The most important reason people chose Ruby is:

Ruby has a very clean syntax that makes code easier to both read and write than more traditional Object Oriented languages, such as Java. For beginning programmers, this means the focus is on the meaning of the program, where it should be, rather than trying to figure out the meaning of obscure characters.
presidents = ["Ford", "Carter", "Reagan", "Bush1", "Clinton", "Bush2"]
for ss in 0...presidents.length
print ss, ": ", presidents[presidents.length - ss - 1], "\n";
end

Pros

Pro

Clean syntax

Ruby has a very clean syntax that makes code easier to both read and write than more traditional Object Oriented languages, such as Java. For beginning programmers, this means the focus is on the meaning of the program, where it should be, rather than trying to figure out the meaning of obscure characters.

Pro

A large ecosystem of tools & libraries

Ruby has a large ecosystem of tools and libraries for just about every use. Such as ORMs (Active Record, DatabMapper), Web Application Frameworks(Rails, Sinatra, Volt), Virtualization Orchestration(docker-api, drelict), CLI tools(Thor, Commando), GUI Frameworks(Shoes, FXRuby) and the list goes on. If you can think of it, there is probably a gem for that ( and if not you can create your own and share with the community).

Pro

Widely used

Ruby is one of the most popular languages for developing web sites. As a result, there's an abundant amount of documentation, sample code, and libraries available for learning the language and getting your project up and running. The most popular features are just 'gem install' away. Additionally, it is easier to find Ruby jobs because of this.

Pro

Ruby on Rails

Lays out an easy to follow and opinionated MVC pattern that teaches best practices through necessity.

Pro

Hugely object oriented

Object oriented programming is one of the most important concepts in programming.

Pro

Test Driven Development, #1

It's the fore-runner and trend setter for TDD.

Pro

Pry

Pro

Newbie-friendly community

Pro

Concise syntax

F#'s syntax tends to be terse while remaining very readable and easy to understand without being a chore to write.

Pro

Easier transition from other paradigms

Since F# is not a purely functional language, it lends itself to being more easily picked up by programmers that have experience with other paradigms.

Pro

.NET Interoperability

Since F# runs on the Common Language Runtime or CLR, it has access to the entire .NET Framework, as well as libraries written in other .NET languages such as C#, VB.NET, and C++/CLI.

Pro

Natively supported by Visual Studio

This is a .Net language natively supported by Visual Studio. Though it is not as tooled up as C# the support is still substantial. In particular, C# deployment scenarios can be enabled for it with small C# wrapper projects. Integration with Visual Studio provides: IntelliSense, debugging, projects an other features.

Pro

Multiplatform, it runs on .Net Core

Forget Xamarin and Mono. F# now runs on the multiplatform .Net Core!

Pro

You can run F# in the browser

Thanks to a project like Fable, you can write and run your client app in F# and run it over JavaScript. It allows you to share code between your server and client.

Pro

F# is supported by Xamarin

Thanks to Xamarin, F# is a functional language that you can use to build for iOS, Android and Windows.

Pro

Fall into the pit of success

F# directs you into a workflow where the right way is the path of least resistance.

Coming from a C# background, its restrictions might feel arbitrary at first (e.g., what do you mean my code has to be in dependency order!? Arbitrary alphabetical or bust!), but you'll soon realize that your code is cohesive, concise and consistent in a way that it never was before - and you can compile and run with confidence!

Cons

Con

Its ecosystem is limited outside of web development

If you're looking to host, generate, manipulate or secure a website, Ruby is your language. There's also some great support here for infrastructure as code work via Chef. However, it just doesn't have the depth and breadth that Python does. Things like native UI development, high performance math, and embedded / small footprint environments are barely supported at all in Ruby-space.

Con

Meta-programming causes confusion for new developers

The ability for libraries to open classes and augment them leads to confusion for new developers since it is not clear who injected the functionality into some standard class.

In other words, if two modules decide to modify the same function on the same class can introduce a number of issues. Mainly, the order in which the modules are included matters. Since you more or less can't tell what kind of "helper" functions a module might write into any class, or for that matter, where the helper function was included from, you may sometimes wonder why class X can do Y sometimes but not at other times.

Con

Monkeypatching

Requiring a library can change the rules of the language. This is very confusing for beginners.

Con

No docstrings

It's hard to access Ruby's documentation from the REPL (irb), unlike Python, Lisp, and Smalltalk which let you ask functions how to use them, which is a great benefit to the beginner, and which also encourages you to document your program as you code it.

Con

Arcane grammar based on Perl

Ruby is too complicated for beginners:

arcane Perlisms;

semi-significant whitespace;

parentheses are not necessary around method arguments, except for sometimes they are;

control constructs could be elegantly implemented with block like Smalltalk (Instead they're baked into the grammar.);

verbose block syntax, unless it happens to be the last argument. (proc lambda).

There are too many exceptional cases and arcane precedence rules.

Con

More than one way to do it

A problem inspired by Perl. The core API interfaces are bloated. There's at least four different ways to define methods. More is not always better. Sometimes it's just more.

Con

Does not teach you about data types

Since Ruby is a dynamically typed language, you don't have to learn about data types if you start using Ruby as your first language. Data types being one of the most important concepts in programming. This also will cause trouble in the long run when you will have to (inevitably) learn and work with a statically typed language because you will be forced to learn the type system from scratch.

Con

Dynamic type system

Majority of bugs could be resolved with types.

Con

Focus on Object-Oriented Programming (OOP)

Focussing on OOP in a beginner stage is an easy and popular plan, but not the best one.

Con

Has no ad-hoc polymorphism (á la Type Classes)

You have generics, you have interfaces, you have inheritance, you have a lot of things at your disposal but you don't have Type Clases. They can be emulated using some clever constructs but there's nothing like having the real thing.